1. Field of the Invention
The present invention relates to a control apparatus for a vibration type driving device which drives a vibration member to produce vibrations, and outputs a driving force by utilizing the vibration energy.
2. Related Background Art
Conventionally, a driving circuit for a vibration wave driving device such as a vibration wave motor or the like is constituted by an analog circuit such as a VCO (voltage-controlled oscillator), a phase shifter, and the like. However, the driving circuit is preferably constituted by a digital circuit to attain a size reduction and a cost reduction of the driving circuit. In an example of the driving circuit realized by a digital circuit, the leading and trailing edge timings of driving pulses as two-phase driving signals for driving a two-phase vibration type (vibration wave) motor are generated based on high-frequency reference clocks, and the switching timing of a switching element on the primary side of a transformer as a booster means is determined using these reference clocks.
When the driving signals are generated by the above-mentioned arrangement, the resolution of the oscillation frequency is limited by the clock frequency. When the speed of the vibration wave motor is to be manipulated based on the frequency alone, smooth speed control cannot be attained. For this reason, the speed must be controlled by manipulating both the frequency and the pulse width of the switching element on the primary side of the transformer.
In order to control both the frequency and pulse width, in Japanese Laid-Open Patent Application No. 64-85587, the speed is controlled by sweeping the frequency from higher to lower frequencies (since the frequency region higher than a resonance frequency in a waveform having the resonance frequency as a peak is used as the control region, the rotating speed increases as the frequency becomes lower in this control region), fixing the frequency when the speed has reached a predetermined speed, and then manipulating the driving voltage or pulse width. The speed at which the frequency is fixed substantially matches the speed when the driving voltage is controlled.
However, in the above-mentioned prior art, when the pulse width upon sweeping the frequency is set at a maximum width upon driving, since the speed at which the frequency is fixed substantially matches the target speed, if some environmental factor such as the load, temperature, or the like changes after the frequency is fixed, the motor can no longer be rotated at the target speed at the fixed frequency.
In order to avoid the above-mentioned problem, the frequency may be swept by setting the pulse width at a value smaller than that which corresponds to the maximum speed. However, since the motor output cannot be maximized upon starting, the rise time is long.
Furthermore, when the target speed is changed to a value that exceeds the variable range of the speed by means of the pulse width (amplitude) during driving by the above-mentioned method, normal driving may be disturbed.
For example, when the target value is changed to a speed which is larger than the upper limit of the variable range of the speed by means of the pulse width (amplitude), the driving speed cannot be raised to the target speed. On the other hand, when the target value is changed to a speed smaller than the lower limit of the variable range of the speed by means of the pulse width (amplitude), the vibration wave driving device enters a dead zone with respect to voltage, and stops.
Furthermore, when the pulse width (amplitude) alone is controlled, the variable range of the speed is narrow, and the target speed cannot be largely changed.